532 PROCEEDINGS OF THE NATIONAL MUSEUM vol. i04 



from 33 to 38 hours of incubation and disappears about the third or 

 fourth day, or by the 32-somite stage; (2) second aortic arch appears 

 at about the end of the second day and at least by the end of 50 to 

 55 hours of incubation, and disappears during the fourth day or by 

 about the 32-somite stage; (3) third aortic (carotid) arch is usually 

 present by the end of the second day or by the end of 50 hours of 

 incubation, and this vessel usually remains throughout the life of the 

 bird, although it may be modified in part in the adult where it forms, 

 at least in part, the common carotid artery; (4) fourth aortic (systemic) 

 arch arises in the embryo during the third day of incubation and is 

 present by the end of 72 hours of incubation; the right arch alone 

 (normally) remains and serves as the functional systemic arch con- 

 necting the systemic root with the dorsal radix aortae on the right 

 side; the left arch is reduced by about QYi days of incubation, and 

 usually entirely obliterated by 7K days of incubation; (5) fifth aortic 

 arch is a transient vessel which makes its appearance during the first 

 half of the fourth day of incubation and disappears by about the end 

 of the fifth day; (6) sLxth aortic (pulmonary) arch makes its appearance 

 usually by the end of the fourth day and persists, at least in part, 

 for the duration of the animal's life; the proximal ends of both vessels 

 remain but become connected with the new pulmonary artery, which 

 forms de novo in situ and supplies the lung; the distal portion atro- 

 phies and the left ductus arteriosus (botalli) usually completely dis- 

 appears, while the right remains in many birds as the ligamentum 

 botalli or it may fuse with the ventral face of the right radix aortae 

 where it appears as a white streak (linea botalli) along the ventral 

 face of the radix; (7) internal carotid artery appears at about the be- 

 ginning of the third day of incubation as an anterior prolongation of 

 the dorsal radLx aortae from which point it extends into the head 

 region, in rather close association with the brain. 



Early changes in aortic arches 



As has already been noted, the first, second, and fifth aortic arches 

 become obliterated at an early stage in the embryonic life of the bird. 

 According to Lillie (1908), these deletions occur on the third and fourth 

 days of incubation in the case of the first two aortic arches, and, as 

 has been pointed out by Hughes (1934), the fifth arch tends to dis- 

 appear during the fifth day of incubation. 



During the sixth to seventh day of incubation the fourth aortic 

 arch of the left side loses its comiection with the truncus. At this 

 same time the dorsal connection between the fourth and third left 

 arch (ductus caroticus) becomes reduced and soon loses its attachment 

 with the left fourth aortic arch. By the 7K-day stage there is no trace of 

 the left systemic arch except in instances of anomalous retention such 

 as those cited by Biswas (1946) and Pohlman (1920). The dorsal 



